1. Field of the Invention
A certain aspect of this disclosure relates to a power supply device and an image forming apparatus including the power supply device.
2. Description of the Related Art
In many electronic devices, appropriate driving power is generated from power supplied from an alternating-current (AC) power supply such as a commercial power supply or a direct-current (DC) power supply such as a battery, and the generated driving power is supplied to circuits on a circuit board. Accordingly, many electronic devices include a power supply device for generating the driving power.
FIGS. 1 and 2 illustrate exemplary configurations including a switching power supply device and an AC power supply. In FIG. 1, a diode bridge 14 and a smoothing capacitor 15 constitute a rectifying-and-smoothing circuit and the rectifying-and-smoothing circuit is connected to a switching power supply device 100. The diode bridge 14 full-wave-rectifies an alternating voltage input from an AC power supply 11. The smoothing capacitor 15 further smoothes the rectified voltage output from the diode bride 14. The smoothed voltage is input to the switching power supply device 100.
In FIG. 2, a power factor correction (PFC) circuit 16 is connected via the smoothing capacitor 15 to the switching power supply device 100. The smoothing capacitor 15 shapes a rectified voltage output from the power factor correction circuit 16 and inputs the shaped voltage to the switching power supply device 100. In either one of the configurations illustrated in FIGS. 1 and 2, a direct voltage is input to a terminal DCin of the switching power supply device 100.
FIG. 3 is a circuit diagram illustrating an exemplary configuration of the switching power supply device 100 according to the related art. The switching power supply device 100 of FIG. 3 includes a transformer T100, a power MOSFET Qsw that is a switching element connected to a primary coil Lp of the transformer T100, a control circuit 101, a voltage detection circuit 201, a diode Dsub, a diode Ds, a starting resistor Rstart, a capacitor Csub (power supply capacitor), and a capacitor Cs.
Power is supplied from a direct-current power supply to a terminal DCin connected to one end of the primary coil Lp and to one end of the starting resistor Rstart. The other end of the starting resistor Rstart is connected to a terminal VCC of the control circuit 101, the capacitor Csub, and one end of the diode Dsub. The control circuit 101 includes a Zener diode ZD1 the forward direction of which is toward the terminal VCC.
When the switching power supply device 100 is started, the direct voltage applied to the terminal DCin causes a starting current Istart of several mA to several tens mA to flow via the starting resistor Rstart into the terminal VCC and the capacitor Csub. The capacitor Csub is charged by the starting current Istart and the control circuit 101 is driven by the power stored in the capacitor Csub. Then, the control circuit 101 starts switching control of the power MOSFET Qsw.
When the power MOSFET Qsw is turned on and off, a current flows through the primary coil Lp of the transformer T100. This current change causes electromagnetic induction and generates an electromotive force in a secondary coil Ls of the transformer T100 and the electromotive force generates a current in the secondary coil Ls. The current generated in the secondary coil Ls is rectified and smoothed by the diode Ds and the capacitor Cs. The rectified and smoothed voltage is output from a terminal DCout of the switching power supply device 100. The diode Ds and the capacitor Cs constitute a first rectifying-and-smoothing circuit 301.
Similarly, when the starting current Istart flows through the primary coil Lp, the current change causes electromagnetic induction and generates an electromotive force in a tertiary coil Lsub, and the electromotive force generates a current. The current generated by the electromotive force is rectified by the diode Dsub connected to the tertiary coil Lsub, charges the capacitor Csub, and is supplied to the terminal VCC to drive the control circuit 101. The diode Dsub and the capacitor Csub constitute a second rectifying-and-smoothing circuit 302.
The voltage detection circuit 201 detects an output voltage to be output from the terminal DCout and generates a feedback signal to be sent to the control circuit 101. The voltage generated by the first rectifying-and-smoothing circuit 301 is divided by resistors RFB1 and RFB2 and a division of the voltage is input to a shunt regulator ZDshunt. The shunt regulator ZDshunt regulates a current flowing through a photodiode PD of a photocoupler PC such that the division of the voltage always equals an internal reference voltage. A resistor RFB5 and a resistor RFB6 regulate the current flowing into the photodiode PD. A capacitor CFB1 and a resistor RFB3 adjust frequency characteristics of the feedback by the photodiode PD.
With the above configuration, when the voltage generated by the first rectifying-and-smoothing circuit 301 exceeds a desired level, a current flows through the photodiode PD of the photocoupler PC. As a result, the photodiode PD emits light and is coupled to a phototransistor PT of the photocoupler PC.
A current flowing through the phototransistor PT generates a voltage, which is determined by the resistance of a resistor RFB4 and the current value, to be applied to a terminal FB of the control circuit 101.
The control circuit 101 turns on and off the power MOSFET Qsw according to the voltage level, at the terminal FB. This configuration makes it possible to control the switching frequency and the time ratio (or duty ratio) of the power MOSFET Qsw according to the output voltage level at the terminal DCout detected by the voltage detection circuit 201 and thereby makes it possible to obtain a desired output voltage.
FIG. 4 is a circuit diagram illustrating another exemplary configuration of the switching power supply device 100 according to the related art. In FIG. 4, a resistor Rocp is connected to one end of the power MOSFET Qsw. Also, one end of the resistor Rocp and the phototransistor PT are connected together to the terminal FB of the control circuit 101. This configuration makes it possible to use the voltage of the current flowing through the phototransistor PT and the voltage of the current flowing through the primary coil Lp for feedback control.
However, the configurations of FIGS. 3 and 4 tend to increase the power consumption. When the switching power supply device 100 is started, the starting current Istart for generating a driving voltage for the control circuit 101 flows via the starting resistor Rstart to the terminal VCC of the control circuit 101 and the capacitor Csub. Also, after the switching control of the power MOSFET Qsw is started, the control circuit 101 is driven by the voltage of the capacitor Csub that is charged by the current flowing from the tertiary coil Lsub via the diode Dsub into the capacitor Csub.
Here, the Zener diode ZD1 is provided at the terminal VCC of the control circuit 101 to prevent damage due to an overvoltage. The Zener diode ZD1 keeps the voltage at the terminal VCC, for example, within a range from several to ten and several volts. Meanwhile, a direct voltage of one hundred and several tens volts to about 400 volts, which is obtained by smoothing an alternating voltage by a diode bridge and a capacitor or by a power factor correction circuit, is applied to the terminal DCin.
Therefore, even after the switching control is started by the control circuit 101 and the capacitor Csub is charged via the tertiary coil Lsub and the diode Dsub, a voltage of one hundred and several tens volts to about 400 volts continues to be applied to the ends of the starting resistor Rstart and the starting resistor Rstart continues to consume power.
Although the load of the switching power supply device 100 is reduced to several watts when it is in a standby state, power loss by the starting resistor Rstart is still large since the power loss is proportional to the square of the voltage across the starting resistor Rstart.
Meanwhile, if the resistance of the starting resistor Rstart is increased to reduce the power loss, the current flowing to the capacitor Csub decreases. This in turn increases the time for charging the capacitor Csub and thereby increases the start-up time of the switching power supply device 100.
To prevent the above problem, a configuration for reducing the power consumption by a starting resistor has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 10-323031). JP10-323031 discloses a power supply device where a starting resistor R1, a switching element Q2, and a capacitor C2 are connected in series to a direct-current power supply in the order mentioned, and the switching element Q2 is driven by resistors R3 and R4 and diodes D2, D3, and D4. In the disclosed configuration, after the power supply device is started, the switching element Q2 is turned off to disconnect the starting resistor R1 from the power supply and thereby to reduce the power consumption.